1. Technical Field
The present invention relates to an antenna mounting printed-circuit board which is incorporated in an equipment having at least a communication function and mounts thereon various modules for realizing various functions.
2. Technical Art
In recent years, for example, like a mobile communication unit such as a cellular phone, or a wireless LAN (Local Area Network) based on the so-called IEEE (Institute of Electronic and Electronics Engineers) 802.11 standard, various wireless communication techniques have been remarkably developed, and in accordance with this, various techniques concerning an antenna element as an inevitably provided member to perform wireless communication have also been developed.
As an antenna element, for example, one in which a radiation electrode, a surface electrode or the like is formed on a cylindrical dielectric is known. This kind of antenna element is generally installed at the outside of an equipment body and is used. However, in the antenna element of such a type that it is disposed at the outside and is used, there are problems that miniaturization of the equipment is obstructed, high mechanical strength is required, and the number of parts is increased.
Then, as an antenna element substituting for this, a chip-like antenna element which can be surface-mounted on a printed-circuit board provided in the inside of an equipment body has been proposed.
As the chip-like antenna element, various ones have been proposed, for example, a so-called reverse F-type antenna in which a conductor as a radiation electrode is formed into a reverse F shape, or a so-called helical antenna in which a conductor is formed into a coil shape. In such a chip-like antenna element, what is formed by using a high dielectric constant material, such as ceramic, as abase member is typical. However, in this kind of antenna element, there are defects that the high dielectric constant material itself is expensive, and the processing thereof is troublesome, and there are problems that the productivity lowers and the manufacture cost increases.
Then, in recent years, with the improvement of a photoetching technique, for the purpose of resolving the disadvantages as stated above, a so-called printed antenna is proposed in which a printed-circuit board having copper foils on both sides is used as a base member, and the photoetching technique is used to form an antenna conductor on this (for example, see patent document 1: JP-A-H05-347509, and patent document 2: JP-A-2002-118411).
The patent document 1 discloses a printed antenna in which an antenna conductor layer including at least a loop-shaped conductor part is formed by using an upper side copper foil of a both-sided substrate, an earth conductor layer is formed by using a lower side copper foil, and an insulating material part between the upper and lower copper foils of the both-sided substrate is used as a dielectric layer. In this printed antenna, a feed part is formed of the copper foil at the side of the earth conductor layer while being insulated from this earth conductor layer, and the loop-shaped conductor part of the antenna conductor layer and the earth conductor layer are connected to each other by a grounding conductor through the dielectric layer. Besides, in this printed antenna, a feeding conductor is made to face the inside of the loop-shaped conductor from the feed part through the dielectric layer, and a series resonant circuit including an inductance element and a capacitor element to cancel reactance of an antenna body part and to broaden the band width is provided between the feeding conductor and the loop-shaped conductor. The patent document 1 describes that by constructing the printed antenna as stated above, the band width can be broadened by using the reactance compensation method, the total combination adjustment after manufacture can be made unnecessary, and a drop in antenna gain can be reduced.
The patent document 2 discloses a helical antenna in which plural through holes are formed alternately in parallel on a printed-circuit board, and ends of these through holes are connected so as to form a spiral as a whole. This patent document 2 describes that a small antenna element for a mobile communication unit can be provided by constructing the helical antenna as stated above.
By the way, in recent years, in the development of an equipment, including a mobile communication unit, for performing wireless communication, importance is attached to miniaturization. Here, consideration will be given to a case where an antenna element is mounted on a printed-circuit board.
The antenna element is mounted through solder or the like on the printed-circuit board on which one or plural modules, such as an RF (Radio Frequency) module, for realizing the function of an equipment body are mounted. Specifically, as shown in a cross-sectional view of FIG. 1, a terminal 210′ of an antenna conductor 210 connected to a power supply 200 is directly welded to a printed-circuit board 220 indicated as broken line parts in the drawing so that an antenna element is mounted on the printed-circuit board 220.
Here, the characteristic of the antenna element can be expressed by a resonant frequency f indicated in a following expression (1). Incidentally, in the following expression (1), L denotes inductance, C denotes capacitance, and π denotes the circular constant.f=1/(2π(LC)1/2)  (1)
Besides, in the antenna element, in the case where the terminal 210′ where a maximum voltage is generated is provided in the vicinity of a ground, since a stray capacitance C′ is generated, a resonant frequency f′ becomes as indicated by a following expression (2). In the antenna element, it is known that the resonant frequency is changed due to the stray capacitance C′, and in accordance with this, the impedance is changed.f=1/(2π(L(C+C′))1/2)  (2)
Here, there is a problem that the antenna element is apt to be influenced by a ground existing in the surroundings, and the characteristic is changed by the existence of the ground. That is, in an equipment for performing wireless communication, for example, like an electrode for grounding, in the case where another metal body 230, as part of various modules, mounted on the printed-circuit board 220 exists to be close to the antenna element, a specified capacitance is eventually generated between the terminal 210′ and the metal body 230. Accordingly, in the equipment for performing wireless communication, the resonant frequency is changed by the occurrence of variation in distance between the terminal 210′ and the metal body 230, and an expected antenna characteristic can not be obtained, and a situation occurs in which the operation is performed as if an antenna element having another characteristic is mounted.
In order to avoid such a problem, in an equipment for performing wireless communication, in general, it has been necessary to design a layout on a printed-circuit board so that the ground, that is, another metal body is not provided in the surrounding area of a place where the antenna element is mounted. In other words, in the equipment for performing wireless communication, for example, as shown in FIG. 2, in order to mount an antenna element 250, it is necessary that a dedicated land 260 in which a ground required by other modules indicated as oblique line parts in the drawing does not exist is provided on a printed-circuit board 270, and the antenna element 250 itself is also designed on the assumption that the ground does not exist in the surroundings. This prevents miniaturization of the equipment, and means that the degree of freedom in the layout on the printed-circuit board is extremely restricted.